1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission which is particularly mounted on an automobile, and more particularly relates to a device to control accumulator back pressure.
2. Description of the Prior Art
Conventionally, a hydraulic device for an automatic transmission has a hydraulic servo for a brake and clutch operatively hydraulically connected with an accumulator so that engaging pressure and releasing pressure are determined by a characteristic of the accumulator to reduce a shift-shock.
Control pressure acts on a back pressure side of the accumulator so that the back pressure is lowered when a throttle opening ratio is small to further reduce shift-shock.
Furthermore, as disclosed in the Japanese Laid-Open patent No. Sho-61-130653, an accumulator control valve is controlled by a solenoid valve which is in turn controlled based on the degree of opening of a throttle valve to reduce shift-shock.
The above-mentioned device having an accumulator control valve applies constant accumulator back pressure to the back pressure side of the accumulator during shifting as shown in FIG. 6(d) and FIG. 7(d) by the dotted lines.
Accordingly, hydraulic pressure applied to the clutch (or brake) rises, as shown beginning at a point A in FIG. 6(c) by the dotted lines, based on a starting resistance offered by an 0-ring. After that, the hydraulic pressure rises linearly at a ratio B until the clutch plates are completely engaged.
As a result, an output torque, as shown in FIG. 6(b) by the dotted lines, rises sharply to point A' corresponding to the point A, and further rises to a sharp peak B' caused by a change in the frictional force at the clutch plates.
Similar to the above, when the hydraulic pressure is released, as shown in FIG. 7(c) by the dotted lines, the clutch hydraulic pressure is lowered, so that the output torque rises sharply as shown in FIG. 7(b) by the dotted lines.
In the above hydraulic device, it is difficult to sufficiently reduce shift-shocks during up-shifting and down-shifting.
Furthermore, a hydraulic control device controlling an accumulator control valve by using a solenoid valve cannot control the back pressure of the accumulator adequately and precisely because the solenoid valve is controlled by the degree to which a throttle is open.